1. Field of the Invention
The present invention relates to a heating device which is applied to an image forming apparatus using a photosensitive material and an image receiving material, and which heats the photosensitive material when images are transferred from the photosensitive material to the image receiving material.
2. Description of the Related Art
A heating device is provided at an image forming apparatus which uses two types of image recording materials, i.e., a photosensitive material and an image receiving material, so as to effect an image recording process. The heating device heats the photosensitive material and the image receiving material in a superposed state and carries out heat development transfer from the photosensitive material to the image receiving material.
As such heating device, a type of heating device is known which includes a cylindrical heating drum in which a heater is accommodated therewithin and heats the circumferential surface thereof, an attaching roller in which a photosensitive material and an image receiving material are superposed and attached to the periphery of the heating drum, and an endless pressing/contacting belt which is disposed at the periphery of the heating drum and trained around a plurality of rollers.
Namely, in this type of heating device, the endless pressing/contacting belt is rotated (traveled) and the heating drum is rotated following the rotation of the endless pressing/contacting belt. In this state, the photosensitive material and the image receiving material are superposed by the attaching roller and trained around the outer periphery of the heating drum. Further, as the photosensitive material and the image receiving material are heated in this state, images can be heat-development-transferred from the photosensitive material to the image receiving material.
On the other hand, in addition to such type of heating device, a heating device is invented which includes a pair of endless belts which are trained around a plurality of rollers and are disposed so as to oppose each other in a pressed and contacted state, and a heating plate which is disposed within the loops of these endless belts and heats the photosensitive material and the image receiving material via the endless belts.
Namely, in this type of heating device, when the photosensitive material and the image receiving material are positioned at the upstream sides of the endless belts in the conveying directions thereof, the endless belts are rotated in a predetermined direction, and the photosensitive material and the image receiving material are taken up between the endless belts so as to be superposed. In this state, the photosensitive material and the image receiving material are conveyed. Next, when the photosensitive material and the image receiving material are conveyed via the endless belts to the position which opposes the heating plate, the rotations of the endless belts are stopped. In this state, the photosensitive material and the image receiving material are heated by the heating plate for a predetermined period of time. In this way, heat development transfer is carried out and images can be formed on the image receiving material.
In the above-described type of heating device in which the photosensitive material and the image receiving material are conveyed by the pair of endless belts, there is a drawback in that the time required to heat-development-transfer from the photosensitive material to the image receiving material is long. Namely, in this type of heating device, the photosensitive material and the image receiving material are heated by the heating plate via the endless belts. Further, when the endless belts are separated from the heating plate due to the rotations of the endless belts, the endless belts are cooled, such that the endless belts need to be heated again at the time of heating the photosensitive material and the image receiving material.
In order to solve the problem, for example, a method is considered in which a preheating plate whose surface temperature is higher than that of a main heating plate for heat development transfer is disposed at the upstream side of the heating plate in the conveying directions of the photosensitive material and the image receiving material. In accordance with this method, while the photosensitive material and the image receiving material are conveyed by the endless belts to the position which opposes the heating plate, the endless belts are heated in advance by the preheating plate, and additionally, the photosensitive material and the image receiving material are heated in advance. Consequently, heat development transfer of the photosensitive material and the image receiving material can be carried out in a short time. However, because the preheating plate is required in addition to the main heating plate, a drawback arises in that the heating device becomes expensive. Further, in this type of heating device, because a greater number of parts such as a pair of belts, a plurality of rollers around which these belts are trained, a plurality of heating plates, and the like are required, the overall cost of the heating device becomes high.
As compared to such heating device, in the type of heating device which uses the heating drum, since the photosensitive material and the image receiving material are directly trained around the outer periphery of the heating drum, there is no drawback such as the time required for heat development transfer becoming long. However, in this type of heating device, because the surface of the heating drum around which the photosensitive material and the image receiving material are trained is curved, in order that the photosensitive material and the image receiving material are properly trained around the outer periphery of the heating drum, it is necessary that the entire surfaces of the photosensitive material and the image receiving material, in particular, the end portions thereof, are pressed invariably against the heating drum. As a result, a greater number of parts such as a plurality of rollers, endless pressing/contacting belts, and the like are needed, and the cost of the heating devise becomes high.